* Josh Poimboeuf [off-list ref] wrote:

quoted

So that's still incomplete in that doesn't analyze the 32-bit build yet, right?

We could do INT3s on 64-bit and NOPs on 32-bit.

Or, possibly even better, we could just keep NOPs everywhere and instead
make objtool smart enough to detect function fallthroughs.  That should
be pretty easy, actually.  It already does it for C files.

Something like the below should work, though it's still got a few
issues:

  a) objtool is currently disabled for crypto code because it doesn't
     yet understand crypto stack re-alignments (which really needs
     fixing anyway); and

  b) it complains about the blank xen hypercalls falling through.  Those
     aren't actual functions anyway, so we should probably annotate
     those somehow so that objtool ignores them anyway.

I'm a bit swamped at the moment but I can fix those once I get a little
more bandwidth.  I at least verified that this patch caught the crypto
missing RETs.

Great, I'd be perfectly fine with such an approach.

Also, if we have that then we could re-apply Alexey's patch and switch to INT3 
(only on 64-bit kernels) without any trouble, because objtool should detect any 
execution flow bugs before the INT3 could trigger, right?

I.e. any INT3 fault would show a combination of *both* an objtool bug and a 
probable code flow bug - which I suspect would warrant crashing the box ...

Thanks,

	Ingo